The Yantar is capable of tampering with them, says Igor Sutyagin, a London-based expert on the Russian military.
But there is no evidence that it has done so.
"It's difficult to tap into optical fibres - it's just light inside, not electrical data," he told the BBC.
"It would be easier just to cut the cable."

The 108m-long (354ft) vessel has a crew of 60 and went into service in 2015.
It was built in the Baltic port of Kaliningrad, the first in a series called Project 22010.
A second, called Almaz, will soon be on its way to the navy.

The Yantar can deploy the three-man submersibles Rus and Konsul, which can dive to about 6,000m (20,000ft).
The Rus submersible has a crew of three and dived to 6,180m in the Atlantic

The Rus submersible has a crew of three and dived to 6,180m in the Atlantic

photo Russian Defence Ministry

Spying on the US

The Russian parliamentary report noted that in the summer of 2015 the Yantar had deployed near the US Kings Bay naval base in Georgia.
"According to Pentagon officials, the Russians were gathering intelligence on US submarines' equipment, including underwater sensors in the DoDIN network," the Russian report said.
Mr Sutyagin said the US underwater sensors near Kings Bay would be interesting for the Russian military, which might wish to copy the US technology.

In late 2016 the Yantar was found to be loitering over undersea communications cables off the Syrian coast, including some links to Europe.
The Yantar's frequent stops at points along a cable route suggested that a submersible was examining the sea floor, the report said.

According to Argentina's Navy document,the last known position of the ship on November 15 was 46°44 ‘ south, 59°54’ west.Mystery of ARA San Juan

Search and rescue

Besides the Argentina mission, the Yantar has been used previously for search and recovery.
The Russian parliamentary report said the ship had located two Russian fighter jets - a Su-33 and a MiG-29 - that crashed into the Mediterranean in 2016, during the Syria war.
The Yantar "recovered secret equipment from the planes in good time", the report said.
That could be secret radar or missile data, or the "identification, friend or foe" system, Mr Sutyagin said.

In 2000 a ship like the Yantar might have saved some Kursk sailors' lives and literally salvaged the Russian navy's reputation.